120 compression

[mvent]

Hi Mike,what would the compression be,and cranking compression on a Jims 120 with 91cc chambers and a 47 degree intake valve close with 0.040 head gasket?
Thanks for your help
Mike V

[Mike]

Using your numbers plus 4cc for valve pockets, area above the top ring and head gasket diameter the computer model shows 10.5 mechanical compression and 9.4 based on the 47 degree intake event. With 9.4 I would expect around 205 cranking pressure but this can vary dramatically on intake system, motor size, stroke, cam duration and overlap (even battery voltage and how fast the motor spins).

[mvent]

Thank you Mike,it's 4.125 x 4.5 stroke standard Jims with CNC ported heads,intake duration is 246, 600 lift,overlap 43, 91cc chambers and 57mm Kuryakyn throttle body.
Thanks in advance for sharing your knowledge.
Mike V

[Mike]

Keep us advised on how it works out!

[mvent]

Hi Mike,finally put some miles on this new engine and did a ccp test,man you were close,it has 208 in front cylinder and 210 in rear,higher than I wanted but with map purchased from you it runs great,and plugs show good fuel mixture,and no detonation.
Hopefully,it will run as well in summers heat here,temp was 40 while riding.
Thanks again
Mike V

[Mike]

Thanks for the update!